Observation of surface ordering of alkyl side chains in polystyrene/polyelectrolytes diblock copolymer Langmuir films

Block polyelectrolytes P(S-x-b-VP/RXy) composed of polystyrene (PS) blocks (x = 200, 260) and alkylated (R = C-n) poly(vinylpyridine) (PVP) (X = I or Br; y = 119-270; n = 4, 10, and 18) have been studied using the Langmuir fi lm balance technique and in situ X-ray and neutron reflectivity measurement s. Previous studies showed that the self-assembled surface micelles (with P S core and RPV+PX- corona) at the air/water interface revealed a characteri stic transition (when n > 6) without any significant hydration or the subme rsion of the corona chains. Combined X-ray and neutron reflectivity studies of P(S-x-b-VP/RXy) polyelectrolytes show that although the polyelectrolyte block is water soluble, it remains adsorbed to the water surface. The thic kness of the adsorbed layer measured by X-ray reflectivity is in agreement with that measured using neutron reflectivity. This agreement indicates tha t the counterions are closely associated with the alkylated PVP chains, loc alized at the air/water interface, and are not distributed into the subphas e. Reduction of the water subphase surface tension (using 1-5 vol % n-butan ol) yields no change in the two-dimensional ordering of the surface micelle s but increases the thickness of the corona layer to ca. 100 A during film compression, indicating that the alkylated blocks become submerged into the subphase under these conditions. In situ Fourier transform infrared spectr oscopy performed at the air/water interface establishes that for n = 10 and 18, the alkyl chains adopt an ordered, all-trans state. However, no orderi ng is observed for n = 4. These results clearly indicate that transition ob served in the pi -A isotherms is related to an order-disorder process of th e alkyl side chains.